The invention relates to a microwave pulse generator, in particular a microwave pulse generator for producing microwave pulses in the nanosecond range.
Microwave pulse generators of this type are used in radar systems, in particular in pulsed radar systems, for exact distance measurements.
Such pulse generators should satisfy the following requirements:
a) The pulse duration of the microwave pulse has to lie in the range of one nanosecond, so that the pulsed radar can meet the requirement for high resolution.
b) The pulsed radar method requires a high coherence of the microwave pulse carrier frequency and the pulse repetition rate. To meet this requirement, an oscillator has to start oscillating always with the same initial phase.
c) The ratio of pulse period to pulse duration has to be large so that the following microwave pulse is transmitted only after all echoes from the target have been received.
d) The energy in the microwave pulse generator must decay quickly after a pulse, i.e., the transition from the pulse duration to the pulse pause has to occur quickly in order to capture very small echoes. The dynamic range of a pulse radar is typically between approximately 80 to 100 dB. The microwave pulse has to have decayed by this amount within a very short time. The RC resonator which determines the frequency, therefore has to have a small Q-value.
Conventional CW microwave oscillators are not adapted to generate coherent pulse trains, because they typically have resonant circuits with a high Q-value (Q greater than 100) to maintain a sufficiently stable frequency. As a result, the settling time of such oscillators will be larger than 100 periods after being switched on, and then once again more than 100 periods until the oscillations have decayed after the oscillator is switched off. Consequently, special microwave pulse generators are very complex and produce a very broad spectrum from which the desired signal is filtered out with band pass filters. Disadvantageously, a microwave pulse generator of this type has a poor efficiency.
The German patent DE 44 01 350 C1 discloses a method for generating microwave pulses and a corresponding apparatus with a step recovery diode (SRD). The method and apparatus described therein satisfy the aforedescribed requirements (a) to (d), but have, as also discussed above, a very poor efficiency. The arrangement disclosed in the reference includes a pulse generator with a pulse differentiating circuit following the pulse generator. A subsequent matched network transmits the pulses to the step recovery diode which is integrated with a resonator. The resonator includes a capacitive and an inductive TEM line section together with the blocking capacitance of the step recovery diode. This arrangement already defines the center of the produced spectrum. Additional filtering with a band pass produces a microwave pulse with the carrier frequency fT and the pulse duration tp.
It is an object of the present invention to provide a microwave pulse generator which requires few components and produces a microwave pulse with a pulse duration tp in the range of one nanosecond. It is another object of the invention to improve the efficiency of the microwave pulse generator by at least a factor of 10 over conventional pulse generators.
According to one aspect of the invention, the microwave oscillator includes an active semiconductor amplifier with a resonant circuit which determines the frequency, and an ohmic device, wherein the ohmic device is provided to specifically reduce the resonant Q-value of the resonant circuit in such a way that a control pulse applied to an input terminal of the semiconductor amplifier produces at the output terminal of the microwave oscillator a microwave oscillation which approximately follows the time dependence of the control pulse.
Specific embodiments of the invention are recited in the dependent claims.
According to an embodiment of the invention, the complexity of the circuit can be significantly reduced by generating control pulses of suitable duration which can be used to power a microwave oscillator or which can be applied to a control input of a semiconductor amplifier of the microwave oscillator. This arrangement obviates the need for expensive step recovery diodes.
The control pulse in the nanosecond range which determines the duration of the actual microwave pulse, may be produced, for example, with a simple pulse shortening stage.
According to another advantageous embodiment of the invention, a driver stage and/or a de-coupling stage can be used to improve coupling of the pulse shortening stage and the oscillator. If the pulse shortening stage is designed to provide an inverted output signal, then the inverted signal can be compensated by an inverting driver stage. Advantageously, the de-coupling stage can be implemented with a so-called xe2x80x9ccircular stub.xe2x80x9d
According to yet another advantageous embodiment of the invention, the oscillator may be formed as a series resonant circuit, which includes, for example, the gate-drain path of a gallium arsenide FET as a capacitive element. This significantly improves the efficiency of the arrangement.
According to another advantageous embodiment of the invention, the pulse shortening stage may include two bipolar transistors, with the input pulse supplied to the base terminal of the two transistors, wherein the collector-emitter path of the second transistor is connected in parallel with the base-emitter path of the first transistor, and wherein the input pulse is supplied to the second transistor via an RC circuit.